Many countries have some sort of "Competition Commission" whose role is to determine if acquisitions/mergers are bad for either the market or consumers. This might be for monopolistic reasons or other negative issues.
In this case, a platform buying up cross-platform software to eliminate support for competing platforms is clearly causing harm to consumers. In similar situations, a government might legislate to force companies to be either the pipe/platform or a content supplier but not both, to eliminate precisely this type of anti-consumer incentive.
I actually don't think it should be legal for a platform to buy software and then remove it from a competing platform. It's incredibly anti-competitive.
If they made their own software and didn't put it on Android, sure. But this isn't it.
You can always come up with a ludicrous policy; it doesn't detract from the potential of regulation. Would you accept General Motors buying freeways and not allowing others' platforms on it? Competition must be regulated.
Also, isn't the barrier between iOS and Android devices artificial? I don't see a veritable technical reason why the two platforms can't be compatible.
Not processors though, right? OSX and Windows are distinct as well, but Web apps are portable. The main reason why there isn't an analogue native standard is that the economics incentivize walled gardens.
It's a crappy decision on their part, no doubt. But saying it should be illegal is way out of line.